Field of the Invention
The invention relates to a method of preparing a titanium and niobium mixed oxide. It also relates to an electrode based on this mixed oxide as well as to a lithium accumulator comprising said electrode.
The field of use of the invention particularly relates to the storage of electric power, be it particularly for portable electronic devices, for hybrid or all-electric cars, but also for the storage of the power originating from photovoltaic cells.
Description of Related Art
The electric energy storage capacity of accumulators, be they of nickel-cadmium (Ni—Cd) and nickel-metal hydride (Ni-MH) or lithium type, provides a significant energy self-sufficiency for the user of electronic devices.
The optimization of such accumulators particularly requires improving their energy density, in other words, a capacity of storing energy per volume and mass unit. In this respect, lithium accumulators are increasingly used, particularly in the field of portable devices, given that their mass and volume energy densities are generally greater than those of Ni—Cd and Ni-MH accumulators.
For example, Li-ion accumulators having an energy density in the order of 200 Wh/kg have been developed. As a comparison, Ni—Cd and Ni-MH accumulators do not exceed 50 and 100 Wh/kg, respectively.
The optimization of such accumulators has also caused the development and the use of new electrode materials.
In the case of the active material of a positive electrode of a lithium accumulator available for sale, it may be a lamellar compound such as LiCoO2, LiNiO2, and mixed compounds Li(Ni, Co, Mn, Al)O2, or a compound having a spinel structure with a composition close to that of LiMn2O4.
However, the active material of the negative electrode generally is carbon (graphite, coke . . . ), the Li4Ti5O12 spinel, or a metal forming an alloy with lithium (Sn, Si, . . . ). The theoretical and practical specific capacities of graphite and titanium oxide negative electrode compounds respectively are approximately 370 mAh/g and 170 mAh/g.
Although it has a lower capacity than graphite, compound Li4Ti5O12 has certain advantages, particularly regarding (i) its high working potential, approximately 1.6 V, which makes it very safe, and (ii) a very good high-rate cyclability.
Other materials have been developed, particularly niobium oxides and titanium and niobium mixed oxides. Niobium indeed has a working potential close to that of titanium. It further enables to exchange 2 electrons per metal (Nb5++2 e−Nb3+).
Among titanium and niobium mixed oxides, ATiNbO5 types (A=H, Li) lamellar oxides, and also Ti2Nb2O9, TiNb2O7, and Ti2Nb10O29, can be mentioned. Some of these compounds have capacities in the order of 250 mAh/g. However, some of these oxides may have the disadvantage of not providing the reversibility necessary to exchanges inherent to lithium accumulators.
Further, the capacity of some of these mixed oxides drastically decreases at a high rate. A niobium doping and a carbon coating may attenuate or even cancel this loss of capacity. However, such techniques require specific experimental conditions particularly difficult to envisage at an industrial scale (vacuum, inert atmosphere).
To overcome these disadvantages, the synthesis of TiNb2O7 by a sol-gel process has been developed. Now, this method previously implies using hydrofluoric acid. Further, although the low-rate performance of the TiNb2O7 compound reaches 280 mAh/g, capacities decrease on use thereof at high rate.
Other syntheses have been developed, particularly that of Ti0.9Nb2.1O7 by a solid process. This material, when coated with carbon, has a capacity of 190 mAh/g at 9 C, that is, 67% of the low-rate capacity. However, such a synthesis also requires constraining experimental conditions (vacuum and inert atmosphere) for the niobium doping and coating steps.
The Applicant has developed a process of titanium and niobium mixed oxide synthesis which requires no experimental conditions such as the creation of vacuum or an inert atmosphere.
The invention thus enables to rapidly prepare a titanium and niobium mixed oxide material having a high-rate behavior improved over prior art oxides.